1. Field of the Invention
The present invention relates to a fixed optical information reading device of optical information reading devices that optically read information.
2. Description of Related Art
There are a handy-type optical information reading device that reads a code in a state where an operator holds the device in his or her hand, and a fixed optical information reading device that is fixed and reads information by moving an object with a code attached. A two-dimensional code reader (hereinafter, referred to as a reader) that reads a two-dimensional code such as a barcode, and a QR code (registered trademark) has been widely spread. One example of the above-described reader is described in JP 2011-76519 A. In JP 2011-76519 A and JP H7-282175 A, provision of a polarization filter in each of an illumination optical system and an imaging optical system is described.    Patent Literature 3: JP H10-287873 A
When a reader is downsized, an optical axis of an imaging element and an optical axis of an illumination system cannot but be made parallel to each other. This is because a distance between the optical axis of the imaging element and the optical axis of the illumination system cannot but be shortened. In the above-described reader, illumination light reflects at a surface of a workpiece (an inspection object product), and regular reflected light enters the imaging element, which makes it hard to read a two-dimensional code. Consequently, the reader needs to be installed with an optical axis of the reader inclined to a normal line of the surface of the workpiece so that the normal line and the optical axis do not coincide with each other. This is referred to as oblique attachment. The attachment of the reader in which the normal line of the surface of the workpiece and the optical axis of the reader are parallel is referred to as front attachment. If the surface of the workpiece is almost planar, inclining the optical axis to the normal line only by a recommended angle enables the two-dimensional code to be read accurately.
In recent years, the two-dimensional code has been printed on a surface (a casting surface) of a casting such as an engine block by laser marking or the like (so-called direct part marking (DPM)). Since as known well, minute irregularities exist on the surface of the casting, the front attachment brings about higher reading accuracy than the oblique attachment. Moreover, since the two-dimensional code is printed in various parts such as a surface (milled surface) of a workpiece subjected to milling, black resin, and a substrate, a proper reading method differs in the respective parts. Thus, a user needs to search for an installation angle suitable for each workpiece.
Moreover, in the oblique attachment, an image obtained by reading the two-dimensional code is distorted, which may cause a reading error. Thus, the front attachment makes it unnecessary to search for the installation angle, thereby reducing an installation load on the user. Moreover, the front attachment has an advantage that the image is not distorted.
As described in JP 2011-76519 A and JP H7-282175 A, the provision of the polarization filters can reduce influence of the regular reflected light even in the front attachment. However, the provision of the polarization filters disables the reader from reading a code provided by the direct part marking on the casting surface. While it can be considered to make the polarization filters detachable, labor for detachment newly occurs.